In general, conventional phase change inks are in the solid phase at ambient temperature, but exist in the liquid phase at an elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the printing medium, they quickly solidify to form a predetermined pattern of solidified ink dots.
More recently impulse inks or bubble jet inks have been disclosed in U.S. Pat. No. 5,021,802. These inks comprise an aqueous sol-gel medium and coloring agents. The inks are thermally reversible sol-gels and exhibit conventional phase change ink behavior such that they are gels (solids) at ambient temperatures and sols (liquids) at elevated temperatures between about 40.degree.-100.degree. C.
An inherent problem associated with aqueous inks employed in drop-on-demand color printers is "feathering" of ink drops after placement onto a substrate such as paper. Feathering is the undesirable nonuniform dispersal or wicking of ink drops along the paper fibers after the inks are placed on the print medium. Feathering may cause intercolor bleeding, poor resolution, and image degradation adversely affecting the print quality. Attempts to resolve this problem include the use of cosolvents to reduce drying time, the use of latex-based inks, the addition of zwitterionic surfactants, the use of pigmented inks, and the use of solid inks. Although solid inks tend not to feather when printed, their implementation significantly increases the cost and complexity of the equipment needed to print such inks.
Aqueous based inks however, suffer from several inherent deficiencies. One especially troublesome problem is the soaking of the ink into paper substrates. This results in poor holdout and ink drop definition, manifesting as "bloom" or "show through" on the back side of the paper. Additionally, the ink tends to run or smear when exposed to water. The lack of gloss in the ink, evident typically in a matte type of finish, also is seen as a deficiency in the use of aqueous inks where bright, glossy images are obtained only by use of specially coated papers. The waterfastness of the inks has been another persistent problem.
These problems are solved by the present invention wherein an aqueous ink utilizes a sulphonated polyester and its copolymers to obtain a high gloss, quick drying, waterfast, and excellent holdout ink.